Add tooling to compare reference and updated output

[jonwis]

Here's some scripts to help validate the comparison. I wish cmd.exe had a better built-in text manipulation tool. Powershell makes this easy; routing through "copy + findstr /v" is very slow.

The comparison looks pretty reasonable on the Big Windows metadata.

[sylveon]

Looks great! I'd suggest adding .ixx to the prepare_versionless_diffs.cmd tool's list of files, as otherwise it misses modules (the current winrt.ixx + future changes which might add more).

Also I hit a footgun with this: I ran build_protection.cmd without rebuilding the cppwinrt binary first when switching branches - suggest either making the script build cppwinrt.exe or mentioning that pitfall in the README.

[sylveon]

Also, I don't think anybody would mind a PowerShell script instead ;)

[jonwis]

Alrighty, done. Powershell and doc update.